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autocomplete user fields (assigned to and CC) on tickets using AJAX

Description

The AutocompleteUsersPlugin allows AJAX completion of users for the owner and CC fields on new and existing tickets. Currently, all the known users are autocompleted though this could be restricted to those that have the ticket owner permission. A /usersURL is served which presents the user data in a format suitable to the javascript autocompleter. The AutocompleteUsersPlugin should gracefully degrade in the case of no JS or the case where [ticket] restrict_owner = true